Corrosion is defined as the chemical or electrochemical reaction between a material, usually a metal, and its environment that produces a deterioration of the material and its properties.
Corrosive attack begins on the surface of the metal. The corrosion process involves two chemical changes. The metal that is attacked or oxidized undergoes an anodic change, with the corrosive agent being reduced and undergoing a cathodic change. The tendency of most metals to corrode creates one of the major problems in the maintenance of aircraft, particularly in areas where adverse environmental or weather conditions exist.
Chromium-based anti-corrosive systems containing hexavalent chromium compounds have proven to be an extremely useful and versatile group of chemistries that are extensively used in aircraft metal treatment processes. They impart many beneficial and essential anti-corrosive characteristics to metallic substrates on which they are applied and have been used extensively for the pre-treatment of metals before coating, adhesive bonding and surface finishing.
Chemically, chromium-based anti-corrosive systems have involved the combination(s) of hexavalent chromium (e.g., CrO3, CrO42−, Cr2O72−) and hydrofluoric acid (HF) in the case of aluminum and its alloys. The hydrofluoric acid removes oxide film from the surface of the metallic substrate (e.g., aluminum) and the hexavalent chromium reacts with the exposed metal and a trivalent chromium oxide precipitates. Using aluminum as an example:Cr2O72−+2Al0+2H+→Cr2O3.H2O+Al2O3 
Chromium oxide such as that produced according to the above reaction is quite useful in anti-corrosive applications. It is quite stable in alkaline environments, it is water repellant (hydrophobic) and may act as a barrier coating towards water. Finally, it exhibits a “self-healing effect”—that is, residual hexavalent chromium in the coating may react with damaged areas of the coating—thereby producing more trivalent chromium oxide at damaged sites and therefore “healing” itself.
Consequently, chromium-based, and in particular hexavalent chromium-based systems have been extensively used in the aircraft industry because they have proven to be: highly effective at preventing corrosion and as an adhesion promoter for organic coatings and adhesives; particularly resilient as the application/treatment process exhibits a low sensitivity towards variation in process conditions; extremely effective on most/all aluminum alloys; and ensure considerable quality control characteristics as a skilled worker may tell the amount of chromium on the surface of a substrate by mere inspection (color) of the coating.
Concern about chromium—and in particular hexavalent chromium—in the environment has generated a need to replace chromium-based systems. Hexavalent chromium salts are classified as hazardous substances (toxic, sensitizing and carcinogenic) consequently they are environmentally and toxicologically undesirable. The European Parliament has published directives requiring the elimination of hexavalent chromium such as directive 2002/95/EC for electrical and electronic equipment and directive 2000/53/EC for the automotive sector. Therefore “environmentally friendly”, commercially acceptable alternative to chromium-based systems are highly desirable.
Prior art attempts to provide chromium-free coatings have met with limited success. For example, R. J. Racicot and S. C. Yang describe and compare the corrosion resistance performance of a polyaniline based conductive polymer coating versus a chromate conversion coating on two aluminum alloys in a paper entitled “CORROSION PROTECTION COMPARISON OF A CHROMATE CONVERSION COATING TO A NOVEL CONDUCTIVE POLYMER COATING ON ALUMINUM ALLOYS”, which was presented at CORROSION 97, paper 531, pp. 531/1-531/7, Houston, Tex., 1997. As disclosed by the authors, the double strand polyaniline exhibited limited corrosion protection for aluminum alloys AA2024-T3 and AA7075-T6 in salt-spray and salt and acid immersion tests.
The double strand polyaniline employed is a molecular complex of two polymers, polyaniline and a second polyanion. The two linear polymers are bonded non-covalently in a side-by-side fashion to form a stable molecular complex. As noted by the authors, the advantages to such double strand complexes is: 1) that the conductive state of the polymer is very stable; 2) with proper choice of the polymeric dopant, the conductive polymer may be dispersed in solvents and used as a coating material; and 3) the polymeric dopant provides sites for functionalization to achieve good adhesion to metal surfaces.
I. Paloumpa, A. Yfantis, P. Hoffmann, Y. Burkov, D. Yfantis and D. Schmeiber describe, in a paper entitled “MECHANISMS TO INHIBIT CORROSION OF Al ALLOYS BY POLYMERIC CONVERSION COATINGS”, which appeared in Surface and Coatings Technology, 180-181, pp. 308-312, 2004, describe a polypyrrole-based coating which can be formed on an aluminum surface from an aqueous polypyrrole (PPY) chemisorbed on titanium and zinc oxides and exhibits advanced corrosion resistance.
U.S. Pat. No. 5,342,456 to Dolan on Aug. 30, 1994, describes a “PROCESS FOR COATING METAL SURFACES TO PROTECT AGAINST CORROSION” wherein a chromium-free conversion coating can be formed on metals—particularly galvanized steel, by dry-in-place aqueous acidic liquids. The liquid comprises a component of anions, particularly at least four fluorine atoms and at least one atom from a group consisting of titanium, zirconium, hafnium, silicon, and boron and optionally, one or more oxygen atoms. Additional cations from the group consisting of cobalt, magnesium, manganese, zinc, nickel, tin, zirconium, iron, aluminum and copper, a sufficient free acid to give a pH in the range of 0.5 to 5.0 and optionally a compound that will form an organic resinous film upon drying in place.
A “CORROSION RESISTANT ALUMINUM ARTICLE COATED WITH EMERALDINE BASE POLYANILINE”, was described in a U.S. Pat. No. 5,928,795 which issued to Spellane et al on Jul. 27, 1999. The polyaniline used as the coating was a well-known emeraldine base form and is easily formed by the oxidative polymerization of aniline in excess hydrochloric acid by ammonium persulfate followed by treatment with ammonium hydroxide.
U.S. Pat. No. 5,980,723 which issued to Runge-Marchese et al. on Nov. 9, 1999, describes an “ELECTROCHEMICAL DEPOSITION OF A COMPOSITE POLYMER METAL OXIDE”, which is a process for forming polymer films through electrochemical techniques utilizing electrolytes which include conductive polymer. The resulting polymer films described are electrically conductive and corrosion and wear resistant. Example polymer films included polyaminobenzene (polyaniline).
An aqueous liquid surface treatment composition having a pH value not more than 6.5 and containing phosphoric acid ions, condensed phosphoric acid ions, an oxidizing agent and a water-soluble polymer was described in U.S. Pat. No. 6,153,022 which issued to Yoshida on Nov. 28, 2000. The patentee therein reports that such coating rapidly forms on the surface of a metal, a conversion coating that has good corrosion resistance and adhesion to subsequently applied organic coatings such as paint and is less easily damaged by mechanical stresses than prior art conversion coatings.
“ELECTROACTIVE POLYMER COATINGS FOR CORROSION CONTROL” were described in U.S. Pat. No. 6,150,032, which issued to Yang et al. on Nov. 21, 2000. In that patent, the patentees describe an anti-corrosive polymeric complex which comprises a plurality of double-stranded molecular complexes including conductive polymer and a strand of a copolymer. The strands of the polymeric complex are non-covalently bonded to each other along the contour of the strands to form a side-by-side, twisted, double-stranded configuration.
U.S. Pat. No. 6,328,874, issued to Kinlen et al on Dec. 11, 2001 for “ANODICALLY FORMED INTRINSICALLY CONDUCTIVE POLYMER-ALUMINUM OXIDE COMPOSITE AS A COATING ON ALUMINUM”, describes a method for forming a coating on aluminum by contacting the aluminum with water, at least one multifunctional polymeric organic acid, a monomer of an intrinsically conductive polymer (ICP) and polymerizing the ICP monomer and forming aluminum oxide by imposing an electrical potential between the aluminum surface as an anode and a cathode. The intrinsically conductive polymer salt and aluminum oxide coating that is formed resists corrosion and is resistant to de-doping during immersion in hot water.
A “NONCHROMATE RUST PREVENTIVE AGENT FOR ALUMINUM, METHOD OF RUST PREVENTION AND RUST-PREVENTIVE ALUMINUM PRODUCTS” was described in U.S. Pat. No. 6,419,731, which issued to Inbe et al. on Jul. 16, 2002. The patentees therein describe a nonchromate rust preventive agent for aluminum that comprises a zirconium compound, a fluoride ion, a water soluble resin and an aluminum salt.
Sako et al., in U.S. Pat. No. 6,736,908, entitled “COMPOSITION AND PROCESS FOR TREATING METAL SURFACES AND RESULTING ARTICLE”, which issued on May 18, 2004, describes a metal treating composition comprising at least a specific type of dissolved and/or dispersed organic resin, a dissolved vanadium compound in which the valence of the vanadium is from 3 to 5, and a dissolved compound that contains at least one of the metals Zr (zirconium), Ti (titanium), Mo (molybdenum), W (tungsten), Mn (manganese), and Ce (cerium). According to the patentees, the treatment provides metal surfaces with superior corrosion resistance, alkali resistance, and fingerprint resistance. Advantageously, their composition contains no chromium.
U.S. Pat. No. 6,758,916 for “COMPOSITION AND PROCESS FOR TREATING METALS”, issued to David McCormick on Jul. 6, 2004, describes a chromium-free conversion coating at least equivalent in corrosion protective quality to conventional chromate conversions that can be formed on metals, particularly cold rolled steel, by dry-in-place aqueous acidic liquid. The liquid has a pH value between 0.5 and 5.0 and comprises “fluorometallate” anions consisting of at least four fluorine atoms; at least one atom of an element selected from the group consisting of titanium, zirconium, hafnium, silicon, aluminum, and boron, and optionally, one or more of ionizable hydrogen atoms and oxygen atoms; a component of divalent or tetravalent cations of elements selected from the group consisting of cobalt, magnesium, manganese, zinc, nickel, tin, copper, zirconium, iron, and strontium—in very precise relative proportions.
Despite the developments of the prior art, the corrosion resistance imparted by non-chromate type treatments is invariably less than that provided by chromate type methods and agents and has not satisfied practical needs—particularly those in the aircraft industries. The disclosure provides a chromium-free coating which, despite being chromium-free, is capable of providing corrosion protection equivalent to or superior than a chromium-type coating, or at least to provide a commercially viable alternative to known coatings.
Wim J. Van Ooij et al. described, in a paper entitled “Modified silane coatings as an alternative to chromates for corrosion protection of aluminum alloys”, which was published in Silanes and Other Coupling Agents, Vol. 3, pp 135-159, Ed. K. L. Mittal, 2004, bis-(3-triethoxysilylpropyl)tetrasulfide, bis-(trimethoxysilylpropyl)amine and vinyltriacetoxysilane based treatments applied onto AA2024-T3 alloys which also incorporated cerium nitrate, tolytriazole and benzotriazole corrosion inhibitors and silica nanoparticles. Self-healing effects were reported for some of the treatments as well as good paint adhesion performance.
Wim J. Van Ooij et al. described, in a paper entitled “Overview: The Potential of silanes for chromate replacement in metal finishing industries” which was published in Silicon Chemistry, Volume 3, Numbers 1-2 (2006), that bis-(3-triethoxysilylpropyl) tetrasulfide treated 7075-T6 panels did not exhibit any sign of corrosion after 336 hours of salt spray exposure.
It is known that alcohol-based silanes offer a higher corrosion resistance to the water-based silane systems as the higher alcohol content removes more water from the film upon drying and the silanol groups can react more easily to form a cross-linked and denser film. Also, the water-soluble silanes remain more hydrophilic even after drying, so they allow higher ingress of water than the solvent-based silanes. It is therefore important that these low-VOC (volatile organic compound) water-based silanes systems are further modified to increase their corrosion inhibition efficiency. In the publication “Effects of addition of corrosion inhibitors to silane films on the performance of AA2024-T3 in a 0.5M NaCl solution”, by Wim J. Van Ooij et al., which was published in Progress in Organic Coatings, 53 (2005) 153-168, corrosion inhibitors (tolytriazole, benzotriazole and inorganic cerium salts) were added to silane films and their corrosion properties studied in 0.5 M NaCl (sodium chloride) solution. The water-based silane solutions were prepared by mixing Bis[3-(trimethoxysilyl)propyl]amine and Vinyltriacetoxysilane in 2:1 and 4:1 parts by volume and about 5% of this mixture was hydrolyzed with 95% of DI (deionized) water. Silane films when loaded with organic or inorganic inhibitors provided improved corrosion resistance. A scratch cell test confirmed that the cerium salts were also potential inhibitors for adding self-healing capabilities for silane films.
The publication “A comparative study on the corrosion resistance of AA2024-T3 substrates pre-treated with different silane solutions”, by A. M. Cabral et al. that was published in Progress in Organic Coatings, 54 (2005) 322-331, reported a comparative study of AA-2024-T3 pre-treated with three different silane solutions (1,2-Bis(Triethoxysilyl) Ethane, bis-(3-triethoxysilylpropyl)tetrasulfide and γ-mercaptopropyltrimethoxysilane). The silane treated samples were further treated with γ-aminopropyltrimethoxy silane prior to painting them with a polyurethane enamel. The AC (alternating current) impedance results showed that silane films provided protection to the substrate. For a short time, the performance was even better than that conferred by the chromate reference treatment.
J. B. Bajat et al. described, in a paper entitled, “Corrosion stability of epoxy coatings on aluminum pretreated by vinyltriethoxysilane”, which was published in Corrosion Science, 50 (2008) 2078-2084, the electrochemical and transport properties and adhesion of epoxy coatings electrodeposited on aluminum 99.7 pretreated by vinyltriethoxyslane (VTES). It was concluded that 5% solution for 10 minutes provided enhanced adhesion and also improved the corrosion stability of a protective system VTES/epoxy coating. The same authors reported, in a publication entitled, “Corrosion protection of aluminium pretreated by vinyltriethoxysilane in sodium chloride solution”, which was published in Corrosion Science, 52 (2010) 1060-1069, EIS and potential-time measurements of VTES films deposited on A199.5% substrates in 3% NaCl (sodium chloride) medium exposure. It was shown that the concentration of VTES had a great influence on the corrosion behavior and morphology of the VTES films while curing time exhibited smaller influence of the VTES film properties.
B. Naderi Zand et al. described, in a paper entitled, “Corrosion and adhesion study of polyurethane coating on silane pretreated aluminium”, which was published in Surface & Coatings Technology, 203 (2009) 1677-1681, the effect of the silane pretreatment's pH on the adhesion strength and on the corrosion protection of subsequent polyurethane (PU) coating on aluminum alloy substrate. The practical adhesion of the coating on the substrate was measured in dry, wet and recovered states via pull-off method for desmutted, chromated and vinyltrimethoxysilane (VTMS) pretreated AA1050 aluminum alloy. VTMS resulted in good adhesion performance in dry, wet and recovered states at pH<isoelectric point (IEP). Corrosion protection of PU coating was studied with EIS and salt spray in the presence of silane layer. At pH<IEP protective performance was considerably higher and comparable with that of chromated specimens.
F. Brusciotti et al. described, in a paper entitled, “Characterization of thin water-based silane pre-treatments on aluminium with the incorporation of nano-dispersed CeO2 particles”, which was published in Surface & Coatings Technology, 205 (2010) 603-613, novel thin films of water-based 1,2-bis(Triethoxysilyl)ethane (BTSE) with the incorporation of nano-dispersed CeO2 particles for improved barrier properties. EIS investigations pointed out a better performance for the coatings where the CeO2 particles were nano-dispersed and uniformly distributed in the layer.
U.S. Pat. No. 6,071,566 which was issued to Kevon Brown et al., relates to a method that comprises applying a solution containing one or more vinyl silanes with or more multi-silyl-functional silanes for treating a metal substrate providing corrosion resistance. The method is particularly suitable for use in zinc coated surfaces. The particular preferred vinyl silane is vinyltriethoxysilane (VS) and the preferred multi-functional silane is 1,2-bis(triethoxysilyl)ethane (BTSE).
Accordingly, the disclosure provides for tackling the disadvantages associated with known art, to provide a chromium-free coating with improved adhesion properties, or at least to provide a commercially viable alternative to known coatings.